I purchased an aftermarket Mass Air Sensor on eBay for my late '99 XK8. At the price of less than $30 he was much cheaper than the new, rebuilt or used ones I have seen.
The replacement is an exact fit to the mountings and plug. The only true difference I could see was the "no name" manufacturer and the stem that holds the bulb is about .25 inches shorter but the bulb appears to be the same size. The end quality is not nearly as fine as the OEM but for the price, it is acceptable. I took it for a quick test drive and it works! It does seems to cause an idle of 1000 rpm lower on the tach but the car still drives well.
I was going to post on the interchange parts section but the stem is slightly shorter.

 

If your idle has changed then the new MAF is giving the incorrect reading to the engine ECU. This is not good.


If I were you I would get one from a reputable source and return the ebay one within the return period.

 

The alternative is that the old one has moved the learned values.

Hard reset (battery off & touch cables) will clear those values and force a relearn.

 

I've been down the route of cheap ebay MAFs, with the associated idle problems and in the end swapped it out for a DENOSO unit from a reputable supplier. It was a lot more expensive than the ebay unit but a lot cheaper than the one from Jag.

 

I've heard many negatives, even horror stories of cheap Ebay copies for these and 02 lambda sensors myself.

Sometimes ( not always ) but sometimes going cheap on certain parts can cost more in the long run

 

Thanks guys for all the feed back. I will replace the cheap MAS with a rebuilt from NAPA for a big $139. Seems foolish for me to place my wonderful Jag at risk to save a few dollars.

 

update
Yesterday I replaced the cheap MAS with the original OEM that I cleaned and let air dry over night. Just to convince myself of the difference I drove the Jag hard with the cheap MAS, the acceleration was noticeably weaker and slower. I repeated the "test" with the clean OEM and the performance I expected was back.
Thanks guys for your help, there is a reason it sells for much less and is no bargain on the XK8.

 

I am no expert, but if your car can idle, at 1,000 RPM lower than with the old unit, would that not indicate that your idle was way too high?